Typical cellular communication systems include base transceiver stations (BTSs) that engage in wireless communication with mobile devices such as cellular phones. An example of such a system is illustrated in FIG. 1. The BTSs 14 of the illustrated system connect to at least one base station controller (BSC) 20 through a local network 16, and transmit and receive phone calls and other data using circuit-switched, time division multiplexed communications protocols, virtual circuit, asynchronous transfer mode (ATM) protocols, and/or other communications protocols. The term “local network” as used herein refers to a network served by a particular BSC 20. The local network 16 is typically an Internet protocol (IP) network, and can generally be considered part of a wider communication network having other portions which can include, for example, other local cellular networks and/or other types of networks such as the Internet. The other network portions can be referred to, with respect to the local network 16, as “outside” network portions. The local network 16 communicates to the outside network portions through a gateway 18.
FIG. 2 illustrates an example of a gateway 18 for use in the cellular communication system of FIG. 1. The gateway 18 includes an interface 40 for communicating with outside network portions, an interface 42 for communicating with the local network 16, a processor 44, and a data storage device 46 which stores information for use by the other components of the gateway 18. The stored information can include, for example, programs for execution by the processor 44.
A mobile device—a/k/a a “mobile unit” (MU) 12—engages in direct wireless communication with one or more of the BTSs 14 in order to ultimately communicate with another end-user device such as another MU or a hard-wired telephone (a/k/a a “land line”). The other end-user device can be within the geographic region served by the local network or can be elsewhere in the wider network—e.g., in an outside network portion.
A typical cellular network—which can include one or more local networks—covers a contiguous area that is divided into multiple cells. Each cell is served by a BTS 14 which provides a wireless link for at least one MU (e.g., a cellular phone) within the cell. The wireless link—which in many systems operates within the radio-frequency (RF) spectrum—is used to transmit electromagnetic data signals representing data being sent between the MU 12 and the BTS 14.
Consider an MU 12 which is engaged in a communication session (e.g., a telephone call). As the MU 12 moves among the cells, the session (i.e., the call) is handed off among the BTSs 14 in order to provide continuous coverage.
Typically, a BSC 20 controls call set-up within the BTSs 14, and inter-cell operations such as handoffs among the BTSs 14. In addition, the BSC 20 in conventional systems generally collects information about the respective BTSs 14 and controls the wireless communication parameters of the BTSs 14, such as transmission strength and modulation parameters. During call handoff, a local handoff controller 806 is used to control the allocation of resources among the other devices—e.g., the BSC 20 and the BTSs 14—which are connected to the local network 16.
For “uplink” communications—i.e., communications sent from a cellular phone or other MU 12—it is common to utilize multiple BTSs 14 to receive data from the MU 12. In conventional systems, the best-quality data signals from one or more of the BTSs 14 are selected by the BSC 20 in order to improve the quality of reception, as is well-known in the art. Typically, the stream of data transmitted from the MU 12 is broken into “frames” (i.e., portions of selected size).
For “downlink” communications—i.e., communications sent from one or more BTSs 14 to the MU 12—multiple BTSs 14 can send signals to a single MU 12 in order to improve the quality of reception, as is well-known in the art.
The above-described functions of: (1) selecting uplink signals received by multiple BTSs 14, and (2) distributing downlink signals through multiple BTSs 14 to a single MU 12, are typically performed by a software and/or hardware system called a “selection and distribution unit” (SDU). The SDU controls various characteristics of the digital transmission of the data to and from each MU. Such characteristics typically include parameters such as frame size and allocation of digital capacity such as bit transmission and processing capacity. In conventional systems, the SDU function is performed by the BSC 20. In addition, the allocation of wireless resources (e.g., wireless bandwidth) to an MU is also performed by the BSC 20. In particular, the BSC 20 also includes a wireless resource allocation function that assigns wireless bandwidth, spreading codes (e.g., Walsh codes), and/or time slots to the respective MUs connected to the local network 16. Moreover, digital transmission parameters such as digital capacity allocation are related to the quantity of wireless resources being used. For example, the digital capacity and the wireless capacity allocated to a particular MU must together increase with increasing data transmission rate. The BSC typically coordinates the SDU function and the wireless resource allocation function such that the allocation of wireless resources matches the allocation of digital resources.
However, the wireless resource requirements of an MU 12 tend to change as the MU 12 moves, and therefore, for optimal effectiveness of communication, it is desirable to update and adjust the allocation of wireless resources among one or more moving MUs. Yet, the BSC 20 is generally at a physical location which is remote from the BTSs 14. Consequently, there is a delay in the transmission, from the BTSs 14, of information regarding MU location. In addition, there is a delay in the transmission of control commands from the BSC 20 to the respective BTSs 14. Therefore, the adjustment of the BTSs 14 tends to lag behind the changes in MU location, resulting in sub-optimal resource allocation and consequent reduction of the efficiency of the wireless communication.
Furthermore, exclusive reliance on a single device—the BSC 20—to perform the SDU and wireless allocation functions increases the probability of loss of all communication channels passing through the BTSs 14 connected to the local network 16, because there is no alternative device which can replace the BSC 20 in the performance of the aforementioned functions. If the BSC 20 fails, the system will lose communication with all of the local BTSs 14. The consequences to users can be severe, because these BTSs 14 typically number in the thousands for a single local network.
Soft handoff techniques which utilize more than one BTS have advantages and disadvantages. For example, in the uplink direction, using more BTSs to receive a signal coming from the MU 12 increases the quality of reception without requiring the MU 12 to broadcast its signal with a high power level. Utilizing a high power level in the uplink direction “steals” capacity from other users and/or cells, because wireless capacity is a function not only of frequency bandwidth but of dynamic range as well. Therefore, in some cases, it can be preferable to use multiple BTSs.
In the downlink direction, using multiple BTSs to transmit a signal to a particular MU 12 can also increase the quality of reception. Such a technique tends to require each of the BTSs 14 to send signals to an increased number of users, however, thereby requiring the BTSs 14 to expend capacity (i.e. bandwidth and/or power) that could otherwise be used to transmit data to other MUs. In particular, if one or more BTSs 14 are required to transmit a wireless signal to a very distant MU—which is more likely to be the case if multiple BTSs are used—the wireless signal must be transmitted using a high power level, thereby putting a large burden on the wireless capacity of the system.
In some cases, two MUs are engaged in a communication session while both are connected, through one or more BTSs, to the same local network. Such conditions can be further understood with reference to FIG. 7d. In conventional systems, data originating from the first MU 706 are transmitted through the airwaves to one or more BTSs 702 and 704, and are then sent through one or more high-capacity uplink communication lines 726 into the local network 16 which sends the data to a BSC 20. The BSC 20 then transmits the data back into the local network 16, from which they are then sent through one or more high-capacity downlink lines 724 into one or more of the BTSs 702 and 704 which transmit the data in the form of wireless data signals to the second MU 720.
On the other hand, if the BTS(s) serving one MU is/are connected to a local network that is separate from that of the BTS(s) serving another MU, then data being transmitted between the two MUs typically passes through a “higher level” device than the BSC 20—i.e., a device serving a wider, broader portion of the cellular network. For example, with reference to FIG. 7d, if the second MU 720 does not have a wireless link to any BTS directly connected to the same local network 16 as the first MU 706, then data originating from the first MU 706 are typically sent through a gateway 18 out of the network 16 where they originated, and received—possibly through a mobile switching center 732—by a network 728 connected to one or more BTSs 730 in wireless communication with the second MU 720. In general, the highest-level device through which the data pass as they travel between the MUs can be referred to as the site of the “call anchor” function. For example, consider a call in which the data transmitted between two MUs 706 and 720 never leave the local network 16 and the devices connected thereto. Data originating from the first MU 706 are received by one or more BTS 702 and/or 704 and are sent through the local network 16 to the BSC 20. The BSC 20 sends the data back through the same local network 16 to one or more of the BTSs 702 and/or 704 connected to the local network 16. The BTSs 702 and/or 704 transmit the data to the second MU 720. Similarly, data originating from the second MU 720 are sent through the BTSs 702 and/or 704 to the network 16, and then to the BSC 20. The BSC 20 sends the data back through the network 16, and then through the BTSs 702 and/or 704, to the first MU 706. In the foregoing example, the BSC 20 would typically be considered the site of the call anchor function 740. Alternatively, if the first MU 706 is linked to the BTSs of a first local network 16, and the second MU 720 is linked to the BTSs of a second network 728, then the call anchor device would typically be a device connecting the two local networks. For example, a mobile switching center 732 can serve as the site of the call anchor function 740.
A disadvantage of performing the call anchor function within a BSC, a mobile switching center, or another device remote from the BTSs is that additional high-capacity communication resources—e.g., additional high-capacity lines or greater transmission capacity within the lines—are required to transmit the data from the sender's BTSs to the call anchor, and back down to the recipient's BTSs. As discussed above, purchase and/or usage of high capacity lines is expensive, and therefore, using a call anchor located remotely from the BTSs can increase the cost of the system by causing additional backhaul load. Yet, conventional systems perform the call anchor function remotely from the BTSs, thereby producing undesirably large backhaul loads.